To understand why, consider again that summer day: If a big,
fluffy cumulus cloud comes drifting by, it's usually good news
for hot cloud-watchers. Low thick clouds cast a refreshing shadow
and reflect sunlight back into space. They cool the planet and
the people beneath them.

On the other hand, high wispy clouds drifting by are less
refreshing. Such clouds cast meagre shadows and, because they
are themselves cold, they trap heat radiated from the planet
below. The air temperature near the ground might actually increase.

It is this schizophrenic behavior that makes clouds so
vexing to researchers who are trying to predict the course of
climate change.

Clouds are an important part of Earth's planetary greenhouse.
Greenhouse gases like carbon dioxide and methane are perhaps
more widely discussed, but clouds can do the same thing: they
warm our planet by trapping heat beneath them. Yet unlike greenhouse
gases, sunlight-reflecting clouds also have a cooling influence.
Furthermore, the air temperature, which is affected by clouds,
in turn affects cloud formation. It's a circular relationship
that makes climate research all the more difficult.

"Clouds remain one of the largest uncertainties in the
climate system's response to temperature changes," laments
Bruce Wielicki, a scientist at NASA's Langley Research Center.
"We need more data to understand how real clouds behave."

Left:
The complex role of clouds in Earth's energy balance. Click
on the image to view an easier-to-read version. Credit: NASA/Langley.

How much sunlight do different kinds of clouds reflect? How
much heat do they absorb? And how do they respond to ambient
temperature changes? Wielicki is the principal investigator for
an orbiting instrument that will answer some of these questions.
"It's called CERES," he says, "short for Cloud
and the Earth's Radiant Energy System."

CERES is a package of three telescopes that watch our planet
from Earth orbit. "One telescope is sensitive to ordinary
sunlight," says Wielicki. "It tells us how much solar
radiation is reflected from clouds or ice." The other two
telescopes sense longer-wavelength infrared heat. They reveal
how much heat is trapped by clouds and how much of it escapes
back to space.

CERES is orbiting Earth now on board NASA's Terra satellite.
The instrument was monitoring our planet last summer when a heat
wave struck California and produced a remarkable surge in infrared
radiation from that region. CERES revealed not only the infrared
glow on the ground, but also how much of that heat was absorbed
by the atmosphere -- key data for global warming studies.

Above: California is glowing in this image of infrared
heat radiating from the Earth. CERES on Terra captured the data
during the 2001 California heat wave.

NASA's Aqua satellite, slated to launch on May 2nd, will soon
carry another package of CERES telescopes to orbit. "Having
CERES on board two satellites (Aqua and Terra) will help us cover
the entire planet -- to study, for example, day-night variations
in Earth's energy balance," explains Wielicki.

CERES is a welcome development for scientists who are often
forced to test their ideas about climate change using computer
models -- models that may or may not faithfully represent our
complicated planet. Using CERES, researchers can now examine
some of those theories in the real world.

For example, a group of scientists recently proposed an idea
called the "iris
hypothesis." They suggested that the canopy of clouds
over the tropical Pacific Ocean recedes when the water's surface
temperature increases. Fewer clouds would open a window through
which heat could escape to space and thus cool the planet. Earth,
they argued, has a natural response that counteracts rising temperatures
-- a bit like an iris in a human eye dilating to adapt to low
light.

But does Earth really respond that way?

Wielicki and other NASA scientists used CERES to test the
idea. It turned out that such clouds did trap infrared heat.
But even more so they reflected visible sunlight back into space.
Fewer of the clouds would mean more global warming, not less.

The iris hypothesis was wrong.

Another
problem CERES will tackle concerns aerosols.
Aerosols are tiny particles like volcanic dust, pollution and
even sea spray suspended in the air. Aerosols reflect sunlight.
They also help clouds form by serving as "nucleation sites"
around which water droplets grow. No one knows if increasing
numbers of aerosols will cool or warm our planet.

"The aerosols are a mess," says Thomas Charlock, a
senior scientist at NASA's Langley Research Center and co-investigator
for CERES. "We don't know how much is out there, and every
gosh-darned aerosol particle looks different from every other
one. So we just can't estimate their influence with calculations
alone."

"What we can do is look at the energy balance in a dusty
area and a non-dusty area," Charlock continues. "That's
where CERES and MODIS (a Terra instrument that can sense aerosol
properties) used together will be very powerful."

Right:
Tracks in Earth's atmosphere left by ... ocean vessels!
The clouds pictured here were created when aerosols from the
ships' exhaust caused moisture in the air to condense into clouds.
[more]

When Aqua joins Terra in orbit, it will bring its own special
set of tools to bear on climate research. Says Charlock: "Part
of our mission we can do much better with [instruments on board]
Aqua -- things relating to humidity and water clouds."

Scientists hope the unprecedented "cloud watching"
power of these two satellites will reveal much about the inner
workings of climate change. Don't expect any pictures of ducks
or dinosaurs, though. Neither satellite has that kind of imagination.
Yet in their own way, they will reveal the complex beauty of
clouds as never before.

Aerosols and Climate Change -- a good article about the questions scientists
have on the role aerosols play in climate change, from NASA's
Earth Observatory

Earthshine: Our planet reflects about 30% of incoming sunlight,
and clouds are responsible for the most
of that reflected light. See a 2 MB mpeg animation
of the Earth's outgoing long-wave heat and its reflected sunlight.

Left: The energy that Earth gains from
incoming sunlight is approximately balanced by the energy reflected
or radiated into space. Click on the image for a detailed
view of Earth's energy budget. Image courtesy NASA.